Create a class in your css. For example:
.x-form-trigger-field{
background-image: url(../images/default/form/image.gif) !important;
} and then call setTriggerStyle method for your trigger field like that: myTwinTriggerField.setTriggerStyle("x-form-trigger-field");

In response to micgala's question, this is slightly different in GXT 3 to account for the appearance pattern.

If replacing a style, image, or template for every single instance, you'll need to make a new <replace-with> tag in your module to describe what is being replaced, and what it is being replaced with. If it is a one-off, it is probably easier to pass the appearance into the constructor of the field/cell.

In the case of TwinTriggerField, use SpinnerField as a guide for how the styles, images can be replaced. If you want two icons, side by side (SpinnerField makes them over/under), it should just be a matter of a little css and an image or two, all packed into a client bundle and passed to the super constructor.